Purity: Doubt about Unwashed Areas After Ghusl
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 363:
If after performing ghusl one realises that he has not washed an area of his body but he does not know which area it is, it is not necessary for him to rewash his head and neck; rather, he must only wash that area of his body that he deems probable he had not washed.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)